Legal tyre tread - confused!

Does anyone know where the tyre tread is measured? My tyres have 3 deep grooves running around the circumference of each tyre and within them are small diagonal "dashes" IYSWIM. Which bit is actually meaured?The large grooves are still fairly deep, but the diagonal dashes have worn away quite a bit.DH has just had his car in for an MOT and passed, but his tyres look similar!